The present invention relates to an improvement in a television video signal A/D converter apparatus for a liquid crystal television receiver.
Recently, portable television receivers using a liquid crystal display panel as a display unit have been developed. In a television receiver using a conventional liquid crystal display panel, a video signal amplified by a video amplifier is converted into a digital signal by an A/D converter, and the digital signal drives the liquid crystal panel. However, the conventional liquid crystal display panel has a narrow gradation range from the white level to black level, and cannot provide an image having good contrast. In order to overcome such a drawback, the average value of the television video signal is operated, and upper and lower reference potentials of the A/D converter are set in accordance with the operated average value, thus obtaining good contrast. Since the video signals do not always change from the white level to black level and the overall range of the video signals need not be converted into digital signals, the conversion range of the A/D converter changes in accordance with the video signals, thus improving contrast. In this manner, when the reference potentials of the A/D converter are set, the video signals are supplied to a low-pass filter so as to detect the average potential a. Thus, the upper reference potential is given as a+.alpha. by adding the average potential a to an appropriate potential .alpha., and the lower reference potential is given as a-.beta. by subtracting an appropriate potential .beta. from the average potential a.
In this manner, conventionally, since the upper and lower reference potentials are obtained by operation, an active element such as an operation amplifier is required, resulting in a complex circuit configuration and high cost. Particularly, in a color television receiver, three circuits for R, G and B are needed, thus considerably increasing the number of elements. In the conventional method, if the video signals do not include DC components, the average potential cannot be detected. For this reason, when the video signals are sent from external equipment, a television signal input terminal and the like must be provided so as to perform DC reproduction processing.